1. Technical Field
This invention relates generally to gray scale printers, and more particularly to a printer in which gray scale is reproduced by modulating the amplitude and duration of a spot exposure source.
2. Background Art
In digital recording apparatus, a record medium is raster-wise exposed with a point light source or sources, such as a scanning laser beam or an array of light-emitting diodes, to form visual or latent images on the record medium. The record medium may, for example, be a photoconductive member on which an electrostatic image is formed by selective exposure to light. The electrostatic image is developable by, say, the electrographic process.
Tone scale image quality depends on both the system tone reproduction curve, which defines a gray scale transformation from the digital image data to the printed image, and on the number of levels of gray the system is capable of reproducing. Halftone imagery is an approximation by the printing apparatus to produce the continuum of gray scale by varying the exposure intensity and/or the printed dot sizes prearranged in a geometric pattern. Variations yield a varying percent of light reflection from the printed image thereby creating the apparent gray scale illusion.
Various proposals have been made for reproducing tones with the use of such apparatus, including modulating the exposure duration and/or the exposure amplitude of the light source. See for example U.S. Pat. No. 4,679,057, which issued to A. Hamada on Jul. 7, 1987. Modulation of the exposure duration is commonly referred to as "pulse width modulation."
In pure pulse width modulation, the current supplied to an exposure source, such as light-emitting diodes or a laser diode, is constant. Gray scale is effected by modulating the "on" time of the light source. Pulse width modulation is used in most commercially available black-only gray scale printers, wherein different pulse widths are required for the different gray levels.
In early apparatus using pulse width modulation, pulses occurred at the start of each allotted pixel time, and lasted for respective time periods to effect the desired gray level, as shown in FIG. 1. In some improved apparatus, pulses are delayed an appropriate period so that the pulses are centered in the pixel time, as shown in FIG. 2.
In a known hybrid laser modulation approach, a scanning laser beam or an array of light-emitting diodes are controlled through combined pulse width modulation and amplitude modulation. Hybrid modulation, as illustrated in FIG. 3, is described in our commonly assigned, copending U.S. patent application Ser. No. 07/787,284, entitled HYBRID DIGITAL IMAGE PRINTER WITH HALFTONE GRAY SCALE CAPABILITY, filed Nov. 4, 1991.
The amplitude modulation aspect of such hybrid printers includes an electronics package generally including an analog-to-digital converter, which requires a finite time period to stabilize. In FIG. 3, such a time period has been provided before the longest pulse begins. Because the pulse is centered, there is also provided a time period at the trailing end of the longest pulse, equal to the time period at the beginning of the longest pulse. This results in an undesirably enlarged pixel time requirement. Even though very fast electronic components, including analog-to-digital converters, are known, fast electronics are very expensive when compared to slower electronics for the same function.
Accordingly, it is an object of the present invention to provide a pulse width modulation scheme in which the slowest, and thereby lowest price, electronic components can be used for a given pixel time.